System and Method For Coupled DPF Regeneration and LNT DeNOx

ABSTRACT

A diesel exhaust aftertreatment system comprises an LNT within an exhaust line. A low thermal mass DPF and a low thermal mass fuel reformer are configured within the exhaust line upstream from the LNT. A thermal mass is configured downstream from the fuel reformer and the DPF, but upstream from the LNT. For LNT denitration, the fuel reformer is rapidly heated and then used to catalyze steam reforming. The DPF is also rapidly heat each time the fuel reformer is heated and the LNT denitrated. The system operates to regenerate the DPF each time the LNT is denitrated. Preferably, a second DPF is provided to augment the performance of the first DPF. Preferably, the first DPF is small and of the flow through type whereas the second DPF is much larger and of the wall flow filter type. The second DPF can be used as the thermal mass.

PRIORITY

This application is a continuation-in-part of U.S. application Ser. No.11/490,913, filed Jul. 21, 2006.

FIELD OF THE INVENTION

The present invention relates to pollution control devices for dieselengines.

BACKGROUND

NO_(x) and particulate matter (soot) emissions from diesel engines arean environmental problem. Several countries, including the UnitedStates, have long had regulations pending that will limit NO_(x) andparticulate matter (soot) emissions from trucks and other diesel-poweredvehicles. Manufacturers and researchers have put considerable efforttoward meeting those regulations. Diesel particulate filters (DPFs) havebeen proposed for controlling particulate matter emissions. A number ofdifferent solutions have been proposed for controlling NOx emissions.

In gasoline powered vehicles that use stoichiometric fuel-air mixtures,NO_(x) emissions can be controlled using three-way catalysts. Indiesel-powered vehicles, which use compression ignition, the exhaust isgenerally too oxygen-rich for three-way catalysts to be effective.

One set of approaches for controlling NOx emissions from diesel-poweredvehicles involves limiting the creation of pollutants. Techniques suchas exhaust gas recirculation and partially homogenizing fuel-airmixtures are helpful in reducing NOx emissions, but these techniquesalone are not sufficient. Another set of approaches involves removingNOx from the vehicle exhaust. These approaches include the use oflean-burn NO_(x) catalysts, selective catalytic reduction (SCR), andlean NO_(x) traps (LNTs).

Lean-burn NOx catalysts promote the reduction of NO_(x) underoxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere isdifficult. It has proven challenging to find a lean-burn NO_(x) catalystthat has the required activity, durability, and operating temperaturerange. Lean-burn NO_(x) catalysts also tend to be hydrothermallyunstable. A noticeable loss of activity occurs after relatively littleuse. Lean-burn NOx catalysts typically employ a zeolite wash coat, whichis thought to provide a reducing microenvironment. The introduction of areductant, such as diesel fuel, into the exhaust is generally requiredand introduces a fuel economy penalty of 3% or more. Currently, peak NOxconversion efficiencies for lean-burn NOx catalysts are unacceptablylow.

SCR generally refers to selective catalytic reduction of NOx by ammonia.The reaction takes place even in an oxidizing environment. The NOx canbe temporarily stored in an adsorbent or ammonia can be fed continuouslyinto the exhaust. SCR can achieve high levels of NOx reduction, butthere is a disadvantage in the lack of infrastructure for distributingammonia or a suitable precursor. Another concern relates to the possiblerelease of ammonia into the environment.

To clarify the state of a sometime ambiguous nomenclature, it should benoted that in the exhaust aftertreatment art, the terms “SCR catalyst”and “lean NOx catalyst” are occasionally used interchangeably. Where theterm “SCR” is used to refer just to ammonia-SCR, as it often is, SCR isa special case of lean NOx catalysis. Commonly when both types ofcatalysts are discussed in one reference, SCR is used with reference toammonia-SCR and lean NOx catalysis is used with reference to SCR withreductants other than ammonia, such as SCR with hydrocarbons.

LNTs are devices that adsorb NOx under lean exhaust conditions andreduce and release the adsorbed NOx under rich exhaust conditions. A LNTgenerally includes a NOx adsorbent and a catalyst. The adsorbent istypically an alkaline earth compound, such as BaCO₃ and the catalyst istypically a combination of precious metals, such as Pt and Rh. In leanexhaust, the catalyst speeds oxidizing reactions that lead to NOxadsorption. In a reducing environment, the catalyst activates reactionsby which adsorbed NOx is reduced and desorbed. In a typical operatingprotocol, a reducing environment will be created within the exhaust fromtime-to-time to remove accumulated NOx and thereby regenerate(denitrate) the LNT.

Creating a reducing environment for LNT regeneration involveseliminating most of the oxygen from the exhaust and providing a reducingagent. Except where the engine can be run stoichiometric or rich, aportion of the reductant reacts within the exhaust to consume oxygen.The amount of oxygen to be removed by reaction with reductant can bereduced in various ways. If the engine is equipped with an intake airthrottle, the throttle can be used. However, at least in the case of adiesel engine, it is generally necessary to eliminate some of the oxygenin the exhaust by combustion or reforming reactions with reductant thatis injected into the exhaust.

The reactions between reductant and oxygen can take place in the LNT,but it is generally preferred for the reactions to occur in a catalystupstream of the LNT, whereby the heat of reaction does not cause largetemperature increases within the LNT at every regeneration.

Reductant can be injected into the exhaust by the engine fuel injectorsor separate injection devices. For example, the engine can inject extrafuel into the exhaust within one or more cylinders prior to expellingthe exhaust. Alternatively, or in addition, reductant can be injectedinto the exhaust downstream of the engine.

U.S. Pat. Pub. No. 2004/0050037 (hereinafter “the '037 publication”)describes an exhaust treatment system with a fuel reformer placed in theexhaust line upstream of a LNT. The reformer includes both oxidation andreforming catalysts. The reformer both removes excess oxygen andconverts the diesel fuel reductant into more reactive reformate.

The operation of an inline reformer can be modeled in terms of thefollowing three reactions:

0.684 CH_(1.85)+O₂→0.684 CO₂+0.632H₂O  (1)

0.316 CH_(1.85)+0.316 H₂O→0.316 CO+0.608H₂  (2)

0.316 CO+0.316 H₂O→0.316 CO₂+0.316 H₂  (3)

wherein CH_(1.85) represents an exemplary reductant, such as dieselfuel, with a 1.85 ratio between carbon and hydrogen. Reaction (1) isexothermic complete combustion by which oxygen is consumed. Reaction (2)is endothermic steam reforming. Reaction (3) is the water gas shiftreaction, which is comparatively thermal neutral and is not of greatimportance in the present disclosure, as both CO and H₂ are effectivefor regeneration.

The inline reformer of the '037 publication is designed to be rapidlyheated and to then catalyze steam reforming. Temperatures from about 500to about 700° C. are said to be required for effective reformateproduction by this reformer. These temperatures are substantially higherthan typical diesel exhaust temperatures. The reformer is heated byinjecting fuel at a rate that leaves the exhaust lean, whereby Reaction(1) takes place. After warm up, the fuel injection rate is increased toprovide a rich exhaust. Depending on such factors as the exhaust oxygenconcentration, the fuel injection rate, and the exhaust temperature, thereformer tends to either heat or cool as reformate is produced.Reformate is an effective reductant for LNT denitration.

U.S. Pat. No. 6,006,515 suggests that a LNT may be regenerated moreefficiently by either longer chain or shorter chain hydrocarbons,depending on the LNT composition and the temperature at whichregeneration takes place. In order to be able to control the selectionbetween long and short chain hydrocarbons, the patent proposes two fuelinjectors, one in the exhaust manifold upstream of the turbocharger andone in the exhaust line immediately before the LNT. Due to the hightemperatures in the exhaust upstream of the turbocharger, fuel injectedwith the manifold fuel injector is said to undergo substantial crackingto form shorter chain hydrocarbons.

During denitrations, much of the adsorbed NOx is reduced to N₂, althougha portion of the adsorbed NOx is released without having been reducedand another portion of the adsorbed NOx is deeply reduced to ammonia.The NOx release occurs primarily at the beginning of the regeneration.The ammonia production has generally been observed towards the end ofthe regeneration.

U.S. Pat. No. 6,732,507 proposes a system in which a SCR catalyst isconfigured downstream of the LNT in order to utilize the ammoniareleased during denitration. The LNT is provided with more reductantover the course of a regeneration than required to remove theaccumulated NOx in order to facilitate ammonia production. The ammoniais utilized to reduce NOx slipping past the LNT and thereby improvesconversion efficiency over a stand-alone LNT.

U.S. Pat. Pub. No. 2004/0076565 (hereinafter “the '565 publication”)also describes hybrid systems combining LNT and SCR catalysts. In orderto increase ammonia production, it is proposed to reduce the rhodiumloading of the LNT. In order to reduce the NOx release at the beginningof the regeneration, it is proposed to eliminate oxygen storage capacityfrom the LNT.

In addition to accumulating NOx, LNTs accumulate SOx. SOx is thecombustion product of sulfur present in ordinarily fuel. Even withreduced sulfur fuels, the amount of SOx produced by combustion issignificant. SOx adsorbs more strongly than NOx and necessitates a morestringent, though less frequent, regeneration. Desulfation requireselevated temperatures as well as a reducing atmosphere. The temperatureof the exhaust can be elevated by engine measures, particularly in thecase of a lean-burn gasoline engine, however, at least in the case of adiesel engine, it is often necessary to provide additional heat.Typically, this heat can be provided through the same types of reactionsas used to remove excess oxygen from the exhaust. Once the LNT issufficiently heated, the exhaust is made rich by measures like thoseused for LNT denitration.

Diesel particulate filters must also be regenerated. Regeneration of aDPF is to remove accumulated soot. Two general approaches are continuousand intermittent regeneration. In continuous regeneration, a catalyst isprovided upstream of the DPF to convert NO to NO₂. NO₂ can oxidize sootat typical diesel exhaust temperatures and thereby effectuate continuousregeneration. A disadvantage of this approach is that it requires alarge amount of expensive catalyst.

Intermittent regeneration involves heating the DPF to a temperature atwhich soot combustion is self-sustaining in a lean environment.Typically this is a temperature from about 400 to about 600° C.,depending in part on what type of catalyst coating has been applied tothe DPF to lower the soot ignition temperature. A challenge in usingthis approach is that soot combustion tends to be non-uniform and highlocal temperatures can lead to degradation of the DPF.

Because both DPF regeneration and LNT desulfation require heating, ithas been proposed to carry out the two operation successively. The mainbarrier to combining desulfation and DPF regeneration has been thatdesulfation requires rich condition and DPF regeneration requires leanconditions. U.S. Pat. Pub. No. 2001/0052232 suggests heating the DPF toinitiate soot combustion, and afterwards desulfating the LNT, wherebythe LNT does not need to be separately heated. Similarly, U.S. Pat. Pub.No. 2004/0113249 describes adding reductant to the exhaust gases to heatthe DPF, ceasing the addition of reductant to allow the DPF toregenerate, and then resuming reductant addition to desulfate the LNT.

U.S. Pat. Pub. No. 2004/0116276 suggests close coupling a DPF and a LNT,with the DPF upstream of the LNT. The publication suggests that thisclose-coupling allows CO produced in the DPF during DPF regeneration toassist regeneration of the downstream LNT by removing NOx during DPFregeneration in a lean environment.

In spite of advances, there continues to be a long felt need for anaffordable and reliable exhaust treatment system that is durable, has amanageable operating cost (including fuel penalty), and is practical forreducing NOx emissions from diesel engines to a satisfactory extent inthe sense of meeting U.S. Environmental Protection Agency (EPA)regulations effective in 2010 and other such regulations.

SUMMARY

One of the inventors' concepts relates to an exhaust aftertreatmentsystem having a lean NO_(x) trap (LNT) within an exhaust line. A firstdiesel particulate filter (DPF) and a fuel reformer, both having lowthermal masses, are positioned within the exhaust line upstream from theLNT. A thermal mass, which can be any device providing a suitably highthermal mass, is positioned downstream from the fuel reformer, butupstream from the LNT. A controller is functional to determine when todenitrate the LNT and initiates the denitration process. During thedenitration process, the fuel reformer rapidly heats and then catalyzessteam reforming. The first DPF also rapidly heats. Each time thecontroller initiates the denitration process, the fuel reformer isheated to steam reforming temperatures and the first DPF is heated to atemperature at which accumulated soot undergoes combustion. The systemis thereby operative to regenerate the first DPF each time the LNT isdenitrated. Implementing this concept improves fuel economy.

Preferably, the system includes an SCR catalyst downstream from the LNT.The SCR catalyst is configured and functional to adsorb and storeammonia generated by the LNT during denitration. Preferably, the systemincludes a second DPF that augments the performance of the first DPF.The first DPF is generally small and of the flow through type while thesecond DPF is generally of the wall flow filter type. The second DPF canbe used as the thermal mass or can be downstream from the LNT.Preferably, the second DPF is regenerated in conjunction with heatingthe LNT for desulfation.

One embodiment of the invention is a method of operating a diesel powergeneration system. A compression ignition diesel engine is operated toproduce a lean exhaust comprising NO_(x) and particulate matter. Theexhaust is treated by passing it through a fuel reformer, a first DPF, athermal mass, and a lean NO_(x) trap. The lean NO_(x) trap traps aportion of the NO_(x) and the first DPF traps a portion of theparticulate matter. A determination is made regarding when to denitratethe LNT. In response to that determination, fuel is injected into theexhaust at a rate that leaves the exhaust lean, whereby the injectedfuel combusts, the fuel reformer heats to steam reforming temperatures,and the first DPF also heats. After heating the fuel reformer to steamreforming temperatures, the LNT is denitrated by making the exhaustentering the fuel reformer, the first DPF, and the lean NO_(x) traprich, whereby the fuel reformer produces reformate that denitrates theLNT. In conjunction heating the fuel reformer and denitrating the LNT,the first DPF is heated to soot combustion temperatures, whereby thefirst DPF regenerates each time the LNT is denitrated. The first DPF hasa low thermal mass that facilitates its being heated to soot combustiontemperatures each time the LNT is denitrated. The fuel reformercomprises a steam reforming catalyst and has a low thermal mass thatfacilitates its being rapidly heated to steam reforming temperatures foreach LNT denitration. The thermal mass can be any device, even a passivestructure, that is functional to substantially reduce the temperaturesto which the LNT during LNT heating and denitration.

Another embodiment of the invention is a diesel power generation systemhaving a compression ignition diesel engine operative to produce a leanexhaust comprising NO_(x) and particulate matter. The system includes afirst DPF that has a low thermal mass and is functional to filter asubstantial portion of the particulate matter from the exhaust; a fuelreformer that has oxidation and steam reforming catalysts and a lowthermal mass; an LNT that is functional to absorb a portion of theNO_(x) from the exhaust and store the NO_(x) under lean conditions andto reduce stored NO_(x) and regenerate under rich conditions; and athermal mass, which has a high thermal mass. An exhaust line channelsthe exhaust from the engine through the fuel reformer and the first DPF,then the thermal mass, and then the lean NO_(x) trap. A fuel injectorinjects fuel into the exhaust line upstream from the fuel reformer andthe first DPF. A controller determines when to denitrate the LNT andcarries out denitration through control over at least the fuel injector.The control is programmed make determinations to denitrate the LNT andcarry out denitrations by controlling fuel injection into the exhaust toa rate that leaves the exhaust lean until the fuel reformer has heatedto steam reforming temperatures, and then to rates that make the exhaustrich until the LNT is denitrated. The fuel reformer is designed andconfigured to be rapidly heated to steam reforming temperatures. Thefirst DPF is designed and configured to regenerate with each LNTdenitration by virtue of being designed and configured to rapidly heatto soot combustion temperatures as the fuel reformer is heated and theLNT is denitrated. The thermal mass is configured and functional tosubstantially reduce the temperatures to which the LNT is heated duringdenitration.

The primary purpose of this summary has been to present certain of theinventors' concepts in a simplified form to facilitate understanding ofthe more detailed description that follows. This summary is not acomprehensive description of every one of the inventors' concepts orevery combination of the inventors' concepts that can be considered“invention”. Other concepts of the inventors will be conveyed to one ofordinary skill in the art by the following detailed description togetherwith the drawings. The specifics disclosed herein may be generalized,narrowed, and combined in various ways with the ultimate statement ofwhat the inventors claim as their invention being reserved for theclaims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary power generationsystem in which some of the inventors' concepts can be implemented.

FIG. 2 is a schematic illustration of another exemplary power generationsystem in which some of the inventors' concepts can be implemented.

FIG. 3 is a plot showing a preferred reformer fuel profile for LNTregeneration.

FIG. 4 is a schematic illustration of another exemplary power generationsystem in which some of the inventors' concepts can be implemented.

FIG. 5 is a schematic illustration of another exemplary power generationsystem in which some of the inventors' concepts can be implemented.

FIG. 6 is a schematic illustration of another exemplary power generationsystem in which some of the inventors' concepts can be implemented.

FIG. 7 is a schematic illustration of another exemplary power generationsystem in which some of the inventors' concepts can be implemented.

FIG. 8 is a schematic illustration of another exemplary power generationsystem in which some of the inventors' concepts can be implemented.

FIG. 9 is a schematic illustration of another exemplary power generationsystem in which some of the inventors' concepts can be implemented.

FIG. 10 is a schematic illustration of an exemplary fuel injector foruse with some of the inventors' concepts can be implemented.

FIG. 11A is a schematic illustration of an exemplary pressureintensifier in a fuel intake configuration.

FIG. 11B is a schematic illustration of an exemplary pressureintensifier in a fuel expelling configuration.

FIG. 12 is a schematic illustration of another exemplary fuel injectorfor use with some of the inventors' concepts can be implemented.

FIG. 13 is a schematic illustration of another power generations system.

DETAILED DESCRIPTION

FIG. 13 is a schematic illustration of a power generation 130. The powergeneration system 130 comprises an engine 9, which is generally acompression ignition diesel engine, a manifold 5, and an exhaustaftertreatment system 131. The power generation system 130 can be partof a diesel-powered vehicle, such as a medium or heavy duty truck. Theexhaust aftertreatment system 131 comprises an exhaust line 16configured to channel exhaust from the manifold 5 through, in order, afuel reformer 12, a first DPF 10B, a thermal mass 13, an LNT 11, asecond DPF 10A, and an SCR catalyst 14. A fuel injector 6 is configuredin inject fuel into the exhaust line 16 upstream from the fuel reformer12 at times and at rates determined by the controller 8.

Alternatively, the DPF 10B can be positioned upstream from the fuelreformer 12 but still downstream from the fuel injector 6. The thermalmass 13, the second DPF 10A, and the SCR catalyst 14 are optional. Thesecond DPF 10A can be positioned upstream from the LNT 11 and in thatposition augment the function of or replace the thermal mass 13. Thethermal mass 13 can be any device providing a suitably high thermalmass. For example, the thermal mass 13 can be an inert, uncoated,monolith substrate. A high thermal mass is high in comparison to that ofthe first DPF 10A and the fuel reformer 12, both of which are designedto be rapidly heated. The fuel injector 6 is optional if the controller8 has another means of introducing fuel into the exhaust upstream fromthe DPF 10A and the fuel reformer 12. Another means for introducing fuelinto the exhaust is, for example, post combustion fuel injection withinone or more engine cylinders.

The exhaust from the manifold 5 is normally lean and typically containsNO_(x), particulate matter (soot), and at least about 4% oxygen. Underlean conditions, the LNT 11 absorbs a portion of this NO_(x). If the SCRcatalyst 14 contains stored ammonia, an additional portion of thisNO_(x) may be reduced therein. A first portion of the particulate matteris trapped by the DPF 10B. A second portion of the particulate matter istrapped by the DPF 10A.

For denitration, the fuel reformer 6 is heated to steam reformingtemperatures by injecting fuel into the exhaust line 16 through the fuelinjector 6 under the control of the controller 8 at rates that leave theexhaust lean. At least a portion of the injected fuel combusts in thefuel reformer 12. Another portion of the injected fuel may combust inthe DPF 10B. If the DPF 10B is placed upstream from the fuel reformer12, at least a portion of the injected fuel combusts in the DPF 10B andanother portion of the injected fuel may combust in the fuel reformer12. After the fuel reformer 12 has reached steam reforming temperatures,as may be determined using a temperature sensor 3, the fuel injectionrate is controlled to make the exhaust condition rich for a period oftime over which the fuel reformer 12 produces reformate and the LNT 11denitates.

As the fuel reformer 12 is heated for denitration, the DPF 10B alsoheats. The DPF 10B may further heat during the rich phase over whichdenitration takes place. The DPF 10B has a low thermal mass, wherebythis heating is substantial and causes the DPF 10B to reach sootcombustion temperatures each time the LNT 11 is denitrated. Typically,heating the DPF 10B from diesel exhaust temperatures to soot combustiontemperatures occurs within about 5-10 seconds or less, which istypically all the time required to heat the fuel reformer 12 anddenitrate the LNT 11. During denitration, the LNT 11 is heated to a muchlesser degree than either DPF 10B or the fuel reformer 12. The thermalmass 13 mitigates heating of the LNT 11 by absorbing and storing heat.Where the DPF 10A is used in place of the thermal mass 13, its massgenerally prevents the DPF 10A from reaching soot combustiontemperatures during LNT denitration. The thermal mass 13 generallyreleases stored heat gradually after denitration.

For desulfation of the LNT 11 and regeneration of the DPF 10A, fuelinjection through the fuel injector 6 is prolonged and the fuel reformer12 is kept at steam reforming temperatures for an extended period. Givensufficient time, the downstream devices will warm. Thereby, the LNT 11can be heated to desulfation temperatures and DPF 10A can be heated tosoot combustion temperatures. The DPF 10A is typically regenerated eachtime the LNT 11 is desulfated, and visa-a-versa.

One of the inventors' concepts is to carry out soot combustion and LNTdenitration simultaneously. FIG. 1 provides a schematic illustration ofan exemplary power generation system 1 configured to implement thisconcept. The system 1 comprises an engine 9 connected by a manifold 8 toan exhaust aftertreatment system 2. The exhaust aftertreatment system 2comprises an exhaust line 16 in which are configured a first injector 6,a DPF 10, a second injector 7, and a LNT 11, in that order with respectto the direction of exhaust flow from the engine 9. A controller 8controls reductant flow through the injectors 6 and 7 using informationfrom the engine 9, and a temperature sensor 3.

The controller 8 may be an engine control unit (ECU) that also controlsthe exhaust aftertreatment system 2 or may include several control unitsthat collectively perform these functions. The controller 8 may havedifferent connections and draw data from different sensors than thoseillustrated in FIG. 1, depending on the control strategy for the exhaustaftertreatment system 2.

The preferred reductant injected by the injectors 6 an 7 is diesel fuel,in which case these are fuel injectors. The advantage of using dieselfuel as the reductant is that it is readily available on diesel-poweredvehicles. Nevertheless, the inventors' concepts extend to systems usingother reductants. Examples of other reductants include gasoline, shortchain hydrocarbon gases, and syn gas.

Instead of the injector 6, a fuel injector for the engine 9 can be used.A diesel engine fuel injector can inject fuel into the exhaust before itleaves the engine. For example, fuel injection can take place during acylinder exhaust stroke. Another alternative is to position the injector6 to inject the reductant into the exhaust manifold 5.

The engine 9 is typically a diesel engine operational to produce a leanexhaust. Lean exhaust generally contains from about 4 to about 20%oxygen. Lean exhaust also generally contains NOx and soot. The engine 9can be operated to reduce the production of either NOx or soot, butreducing the output of one pollutant typically increases the output ofthe other. Typical untreated diesel engine exhaust containsenvironmentally unacceptable amounts of both NOx and soot.

The DPF 10 is operative to remove most of the soot from the exhaust. TheLNT 11 is operative to adsorb and store a substantial portion of the NOxfrom the exhaust, provided the LNT 11 is in an appropriate temperaturerange. Over time, the DPF 10 becomes filled with soot and begins to loseactivity or cause unacceptable backpressure on the engine 9. Also overtime, the LNT 11 becomes saturated with NOx and begins to lose itseffectiveness as well. Accordingly, both devices must be regeneratedfrom time to time.

The DPF 10 is regenerated by heating it to a temperature at which theaccumulated soot undergoes combustion. Combustion is exothermic. If thetemperature of the DPF 10 is sufficiently high, there is sufficient sootloading in the DPF 10, and there is sufficient oxygen in the exhaust,soot combustion is self-sustaining. LNT 11 is regenerated by supplyingit with reductant at a rate that leaves the exhaust rich.

Regeneration of the DPF 10 is begun by heating the DPF 10. The DPF 10 isheated by injecting reductant using the injector 6. At least a portionof this reductant combusts to heat the DPF 10. The combustion may takeplace in the DPF 10, provided the DPF 10 has a suitable catalyst, or thecombustion may take place in another device upstream of the DPF 10, suchas a separate oxidation catalyst. The DPF 10 is heated at least untilsoot combustion initiates. After soot combustion has initiated, it maybe desirable to stop injecting reductant using the fuel injector 6 inorder to slow the rate at which the DPF 10 heats, although in certainconfigurations ceasing reductant injection can actually lead to higherDPF temperatures as discussed more fully below.

LNT regeneration is begun by injecting reductant using the reductantinjector 7. Reductant is injected at a rate that leaves the exhaustdownstream of the injector 7 rich. LNT regeneration may begin while theDPF 10 is being heated, or as soot combustion begins. In either case, aportion of the oxygen in the exhaust will have been consumed upstream ofthe injector 7 either by combustion of soot or combustion of reductantfrom the injector 6.

Simultaneously regenerating the LNT 11 and the DPF 10 can reduce thefuel penalty for regenerating the LNT 11 in at least two ways. One isthat reductant used to heat the DPF 10 can serve a dual use; thereductant heats the DPF 10 and the reductant removes oxygen from theexhaust that must be removed to regenerate the LNT 11. The other way isthat the oxygen removed from the exhaust by soot combustion does nothave to be removed by reductant injection.

This later function is present regardless of how the DPF 10 is heated.Thus, the inventors' concept extends to systems in which the DPF 10 isheated without consuming oxygen from the exhaust. For example the DPF 10can be heated electrically. Once the DPF 10 is sufficiently hot, theinventors' concept can be implemented by injecting reductant using theinjector 7 to make the exhaust rich and regenerate the LNT 11 as soot iscombusting in the DPF 10.

The concept of simultaneous LNT and DPF regeneration is particularlyuseful when the reductant is fuel and the exhaust line 16 comprises afuel reformer 12 upstream of the LNT 11. FIG. 2 is a schematicillustration of an exemplary power generation system 20 comprising theseand other additional components. The additional components include anoxidation catalyst 15, the fuel reformer 12, a thermal mass 13, and aSCR catalyst 14.

The oxidation catalyst 15 is functional to combust reductant from theinjector 6 to generate heat for warming the DPF 10. Optionally, theoxidation catalyst 15 is also functional to convert some NO to NO₂. NO₂can contribute to the regeneration of the DPF 10 even under leanconditions, provided the DPF 10 has an appropriate catalyst. NO₂ mayalso remove carbonaceous deposits from the fuel reformer 12 and the LNT11, be adsorbed more efficiently than NO by the LNT 11, and provide theexhaust with an NO to NO₂ ratio that results in more efficient NOxreduction by the SCR catalyst 14.

The reformer 12 converts injected fuel into more reactive reformate. Anoxidation catalyst could be used in place of the reformer 12, although afuel reformer is preferred. A reformer that operates at diesel exhaustgas temperatures requires a large amount of catalyst and may excessivelyincrease the cost of an exhaust aftertreatment system. Accordingly, thereformer 12 is preferably of the type that has low thermal mass and mustbe heated to be operational.

The thermal mass 13 is another optional component placed upstream of theLNT 11. The thermal mass 13 acts to reduce the magnitude of temperatureexcursion experienced by the LNT 11 due to heat generated in upstreamdevices. Frequent large temperature excursions can reduce the lifetimeof the LNT 11.

The SCR catalyst 14 functions to adsorb and store ammonia generated bythe LNT 11 during rich regeneration phases. During the lean phasesbetween regenerations of the LNT 11, the SCR catalyst uses this storedammonia to reduce NOx slipping past the LNT 11 thus increasing theoverall extent of NOx mitigation.

In the system 20, combustion to heat the DPF 10 and soot combustion inthe DPF 10 reduce the amount of oxygen that must be removed by thereformer 12 in order for the reformer 12 to produce reformate. Inaddition, heat generated by these processes can reduce the amount offuel that must be injected to heat the reformer 12 to an operatingtemperature.

In one embodiment, upon receiving a signal to commence regeneration,fuel injection through injector 6 begins. The fuel combusts in theoxidation catalyst 15, heats the DPF 10 and, to a lesser extent, heatsthe reformer 12. Once the DPF 10 reaches a sufficiently hightemperature, soot combustion begins. Fuel injection through the injector7 can begin at any time, but preferably begins after fuel injectionthrough the injector 6 begins, more preferably at about the time thatsoot combustion begins or shortly thereafter.

If the reformer 12 is not yet warm enough when fuel injection throughthe injector 7 begins, fuel injection through the injector 7 is at arate that leaves the exhaust lean, whereby essentially all of theinjected fuel is combusted to heat the reformer 12. Once the reformer 12is sufficiently warm, the fuel injection rate through the injector 7 isincreased to a point that leaves the exhaust rich, whereupon reformateproduction begins. Fuel injection through the injector 7 is terminatedwhen the LNT 11 has been regenerated to a satisfactory extent. Fuelinjection through the injector 6 can be terminated once the DPF 10 hasreached a temperature where soot combustion is self-sustaining, however,fuel injection through the injector 6 can be continued as long as itdoes not cause overheating of the DPF 10. Preferably, the period overwhich the reformer 12 is producing reformate overlaps the period inwhich soot is combusting within the DPF 10.

In a prior art method, soot combustion in the DPF 10 continues untilthere is no longer sufficient soot to sustain combustion temperatures.According to another of the inventors' concepts, however, sootcombustion can be continued and soot removed to a greater degree. Sootcombustion can be continued by injecting fuel through the fuel injector6 to provide sufficient heat to sustain soot combustion temperatures inthe DPF 10. A fuel injection that had been stopped when the DPF 10 firstreached a sufficient temperature for self-sustaining soot combustion maybe resumed for this purpose. This additional fuel might be consideredunderutilized if LNT regeneration were not simultaneous. Using theinventors' concept, however, this is fuel that would be required in anyevent to continue regeneration of the LNT 11.

The systems 1 and 20 can be configured so that the DPF 10 and the LNT 11are always regenerated simultaneously. However, it is possible toregenerate one device more frequently than the other. The DPF 10 can beregenerated independently of the LNT 11 by using only the injector 6.The LNT 11 can be regenerated independently of the DPF 10 by using onlythe injector 7. In order that the DPF 10 can be heated quickly with alow fuel penalty and in order that a large portion of the heat generatedin the DPF 10 is quickly transported downstream, the DPF 10 preferablyhas a small thermal mass. A small thermal mass is achieved by having asmall size and thin walls. The DPF 10 can be a wall flow filter or apass through filter and can use primarily either depth filtration ofcake filtration. Any DPF with a suitably low pressure drop can be used,but one that uses primarily depth filtration may be more conducive tomaintaining a small thermal mass while keeping engine back pressurewithin acceptable limits.

Cake filtration is the primary filter mechanism in a wall flow filter.In a wall flow filter, the soot-containing exhaust is forced to passthrough a porous medium. Typical pore diameters are from about 0.1 toabout 1.0 μm. Soot particles are most commonly from about 10 to about 50nm in diameter. In a fresh wall flow filter, the initial removal is bydepth filtration, with soot becoming trapped within the porousstructure. Quickly, however, the soot forms a continuous layer on anouter surface of the porous structure. Subsequent filtration is throughthe filter cake and the filter cake itself determines the filtrationefficiency. As a result, the filtration efficiency increases over time.In the prior art, the filter cake was generally allowed to build to athickness from about 15 to 50 μm deep before regeneration began. In thepresent invention, if a wall flow filter is used, regeneration beginsbefore the cake is about 10 μm deep, more preferably before the cakeabout 5 μm deep, still more preferably before the cake is about 2 μmdeep.

For a wall flow filter, a small size is typically about 1/10th theengine displacement or less. Preferably, the size is about 1/20th theengine displacement or less. The diameter of the DPF 10 is preferablyabout the same as that of an upstream or downstream abutting exhaustpipe. Wall flow filters are typically made from ceramics, especiallycordierite or SiC.

In contrast to a wall flow filter, in a flow through filter the exhaustis channeled through macroscopic passages and the primary mechanism ofsoot trapping is depth filtration. The passages may have rough walls,baffles, and bends designed to increase the tendency of momentum todrive soot particles against or into the walls, but the flow is notforced though micro-pores. The resulting soot removal is considereddepth filtration, although the soot is generally not distributeduniformly with the depth of any structure of the filter. Preferably, thefilter has metal walls, which can be made very thin to keep the thermalmass low. Emitec™ produces such filters. A flow through filter can alsobe made from temperature resistant fibers, such as ceramic or metallicfibers, that span the device channels. A flow through filter can belarger than a wall flow filter having equivalent thermal mass

Reducing the size of the DPF 10 generally involves a reduction in sootstorage capacity. This is acceptable in that the DPF 10 can beregenerated much more frequently than a conventional DPF, which would beregenerated much less frequently than the LNT 11. In order to maintainthe functionality of the DPF 10, the DPF 10 must generally beregenerated at least about 20% as often as the LNT 11, more typically atleast about 50%, and still more typically at least about 70% as often.In other terms, the DPF 10 generally needs to be regenerated at leastabout once every 10 minutes, more typically at least about once every 5minutes, still more typically at least about once every 3 minutes.

It is acceptable if the DPF 10 is regenerated more often than necessary,but the above regeneration requirements are indicative of the DPF 10being optimally sized for use in conjunction with the inventors'concepts. Having a somewhat greater capacity in the DPF 10 than in theLNT 11 facilitates a simplified control scheme, where only the criteriafor LNT regeneration is examined by the controller 8, it being assumablethat if the LNT 11 is being regenerated often enough, the DPF 10 isbeing regenerated often enough as well.

If it is difficult to achieve a target level of particulate emissioncontrol while maintaining a sufficiently small size of the DPF 10, oneoption is to install a second DPF downstream of the LNT 11. For example,this second DPF might be used as the thermal mass 13. The second filtercan be of the wall flow type and much large than the DPF 10. Preferably,however, the majority of the particulates are removed by the DPF 10. Thesecond DPF can be heated for regeneration in conjunction with heating ofthe LNT 11 for desulfation.

The time at which to regenerate the LNT 11 to remove accumulated NOx canbe determined by any suitable method. Examples of methods of determiningwhen to begin a regeneration include initiating a regeneration uponreaching a thresholds in any of a NOx concentration in the exhaust, atotal amount of NOx emissions per mile or brake horsepower-hour over aprevious period or since the last regeneration, a total amount of engineout NOx since the last regeneration, an estimate of NOx loading in theLNT 11, and an estimate of adsorption capacity left in the LNT 11.Regeneration can be periodic or determined by feed forward or feedbackcontrol. Regeneration can also be opportunistic, being triggered byengine operating conditions that favor low fuel penalty regeneration. Athreshold for regeneration can be varied to give a trade off betweenurgency of the need to regenerate and favorability of the currentconditions for regeneration. The time at which to regenerate the LNT 11can be determined by the controller 8, which generates a control signalthat initiates the regeneration process.

In addition to the option of carrying out denitration simultaneouslywith soot combustion, the inventors have also conceived the idea ofcarrying out desulfation simultaneously with soot combustion. Thislatter concept can be implemented with systems having the same schematicstructures as the systems 1 and 20 illustrated in FIGS. 1 and 2. Themain differences from the previous description are in terms of the sizeof the DPF 10 and the method of operation.

When implementing the concept of simultaneous soot combustion anddesulfation, the operation of systems 1 and 20 between regenerationsremains the same as previously described. The DPF 10 accumulates sootand the LNT 11 stores a portion of the exhaust NOx. In the system 20,the SCR 14 reduces a portion of the NOx slipping past the LNT 11 usingstored ammonia.

During LNT denitration, the DPF 10 is generally not heated significantlyand continues to accumulate soot. LNT denitration is carried out withreductant injection, which may be carried out using either or both theinjectors 6 & 7. For the system 20, fuel is first injected at a ratethat leaves the exhaust lean and heats the reformer 12, then at a ratethat leaves the exhaust rich, causing reformate to be produced andregenerating the LNT 11.

The LNT 11 can be desulfated independently of regenerating the DPF 10and the DPF 10 can be regenerated independently of desulfating the LNT11, however, preferably the DPF 10 is regenerated each time the LNT 11is desulfated. More preferably regeneration of the DPF 10 anddesulfation of the LNT 11 are always simultaneous.

If desulfation is to be carried out simultaneously with soot combustion,the DPF 10 is preferably large enough to only require soot combustionapproximately as often as the LNT 11 requires desulfation. Aconventionally sized wall-flow DPF may serve this purpose. Generally theLNT 11 must be desulfated at least about 20% as often as the DPF 10needs to be regenerated, more typically at least about 50%, and stillmore typically at least about 70% as often.

It is acceptable if the LNT 11 is desulfated more often than necessary,but the above regeneration requirements are indicative of the DPF 10being optimally sized for use in conjunction with the inventors' conceptof making LNT desulfation simultaneous with soot combustion. If one ofthe DPF 10's storage capacity for soot (in terms of lengths of timesbetween regenerations) and the LNT 11's storage capacity for sulfur isgreater than the other, the frequency of simultaneous regenerations canbe based on the requirements of the device needing the more frequentregenerations.

The times to regenerate the DPF 10 and desulfate the LNT 11 can bedetermined in any suitable fashions. When the DPF 10 is a wall flowfilter, the time to regenerate the DPF 10 can be determined bymonitoring the pressure drop across the DPF 10. Desulfation may bescheduled periodically, e.g., after every 30 hours of operation.Alternatively, desulfation may be scheduled based on an estimate of theamount on SOx stored in the LNT 11. The amount of stored SOx can beassumed to increase in proportion to fuel usage and to decrease in amanner dependent on the extent of desulfations. A further option is todetermine the need for desulfation based on system performance, e.g.,based on the activity of the LNT 11 following an extensive denitrationor based on the frequency with which denitration is required

To initiate soot combustion and DPF regeneration, reductant is injectedthrough the injector 6. The injected reductant combusts, heating the DPF10 and, to a lesser extent, the downstream LNT 11. Eventually sootcombustion in the DPF 10 begins. Soot combustion provides additionalheat the DPF 10.

Heat from the DPF 10 will eventually warm the LNT 11, but rather thanwaiting for this processes, the LNT 11 can be separately heated byinjecting reductant through the injector 7 at a rate that leave theexhaust lean. In the case of the system 20, the rate of this injectionmay need to be limited to avoid overheating the reformer 12.

Once the DPF 10 has reached a sufficient temperature for self-sustainingsoot combustion, the reductant injection through the injector 6 may bediscontinued. Once the LNT 11 has reached a sufficient temperature fordesulfation, the reductant through the injector 7 is increased to a ratethat makes the exhaust rich and causes desulfation in the LNT 11. As inthe case of simultaneous soot combustion and LNT denitration, sootcombustion in the DPF 10 reduces the amount of reductant that must beinjected to consume excess oxygen and thereby reduces the fuel penaltyfor desulfating the LNT 11. Additional fuel is saved in that heat fromthe DPF 10 warms the LNT 11.

Soot combustion during desulfation also promotes stable operation of thereformer 12. As discussed more fully below, the reformer 12 may have atendency to overheat when operated steadily for long periods. Removingsome of the oxygen from the exhaust mitigates this overheating problem.More heat is generated in the DPF 10 by soot combustion to approximatelythe same extent that less heat is produced in the reformer 12 due toless remaining oxygen, and the heat from the DPF 10 tend to betransported to the reformer 12, however, overheating of the reformer 12is nonetheless mitigated due to heat losses to the surroundings and moreuniform heat distribution.

Even when the DPF 10 is sized for simultaneous soot combustion anddesulfation, rather than simultaneous soot combustion and denitration,the injector 6 may be used for LNT denitration. Fuel injected using theinjector 6 that does not fully combust before it reaches the reformer 12becomes better mixed with the exhaust than fuel injected using theinjector 7. The fuel injected with injector 6 also tends to become moredispersed along the direction of flow, which could be a disadvantage ifinjector 6 were the only one used. Combining the two fuel injectors,however, allows a balance between good mixing and precise control ofexhaust air-fuel ratios at the reformer 12. Such a balance is easier toachieve if there is no oxidation catalyst 15 and the DPF 10 has arelatively low catalyst loading whereby a substantial portion of thefuel injected by the injector 6 makes it to the reformer 12.

FIG. 3 illustrates a preferred reductant fuel injection profile fordenitrating a LNT positioned in an exhaust line downstream of a fuelreformer. Line 31 is the fuel injection rate, line 32 is the exhaustexhaust oxygen flow rate (controlled through an engine intake airthrottle), line 33 is the resulting reformer temperatures, and line 34is the resulting reformate flow rate. After an initial heating period,the fuel injection 31 is controlled through an approximately Gaussianprofile, which causes the reformate flow rate 34 to begin relativelylow, increase to a maximum, and then decreases toward the end. This typeof reformate profile has been found to provide superior denitration fuelefficiency in comparison to a constant reformate flow profile. Thesuperior efficiency is in terms of less NOx slip during denitration,more conversion of stored NOx per unit fuel used, and more ammoniaproduction during regeneration.

A theory that explains the functionality of this preferred reductantflow rate or concentration profile is that the reductant supply rateapproximately matches the NOx release rate. At the beginning ofregeneration, reductant is consumed by reaction with oxygen stored inthe LNT 11. Until this stored oxygen is removed, reduction of NOx is noteffective, particularly not deep reduction of NOx to NH₃.

Regeneration does not take place uniformly throughout the LNT 11. Oxygenis first removed near the entrance. The point of oxygen removal isbelieved to form a front that moves towards the exit of the LNT 11. Asthis front moves through the LNT 11, a greater and greater portion ofthe LNT 11 is essentially free of stored oxygen and begins to undergorelease of stored NOx. As this portion of the LNT 11 increases, the NOxrelease rate also increases. By progressively increasing the reductantsupply rate, this release rate can be approximately matched by thereductant supply rate while oxygen is being removed at a relativelyconstant speed. Eventually, after essentially all of the stored oxygenis removed and the NOx release rates in the oxygen-free zones are ebbingdue to depleting reserves of stored NOx, the overall NOx release ratedecreases. By decreasing the reductant supply rate toward the end of theregeneration, the reductant supply rate can be approximately matched tothe NOx release rate in the latter part of the regeneration as well.

A highly dispersed fuel injection from the injector 6 can naturallyprovide a Gaussian profile of the type desired. When a more exactcontrol of the fueling rate is desired, for LNT warm-up or perhaps uponthe transition from lean to rich, the fuel injector 7 can used.Together, the two can provide any desired profile and a balance betweenprecise control of profile shape and excellent mixing of fuel andexhaust.

If there are devices comprising oxidation catalysts between the upstreaminject 6 and the reformer 12, some of the injected fuel will not reachthe reformer 12. The reformer temperatures may be different as a resultof this oxidation, but the reformate production rates will be much thesame in that, excepting the effect on reformer temperatures, it makeslittle difference whether oxidation takes place in the reformer 12 orupstream of it.

The different distribution of heat depending on whether fuel is injectedusing the injector 6 or the injector 7 can be used to stabilizeoperation of the fuel reformer 12. When a fuel reformer of the preferredtype is operated to produce reformate at high exhaust oxygenconcentrations, e.g., 8-14%, there is a tendency of the reformer 12 tooverheat. In principle, overheating could be reduced by increasing thefuel injection rate, which would be expected to increase the rate ofendothermic reaction (2) while the rate of exothermic reaction (1)remains constant. In practice, however, the reformer 12 and the LNT 11generally cannot operate efficiently with such high fueling rates. Analternative solution is to pulse the fuel injection to the reformer 12,allowing the reformer 12 to cool between pulses. Disadvantages to fuelpulsing include loss of efficiency due to reductant from rich phasesreacting with oxygen from lean phases.

The structure illustrated in FIG. 2 provides a different way to controlheating in the fuel reformer 12. Even if the DPF 11 is not beingregenerated, a portion of the fuel required to make the exhaust lean andproduce a target amount of reformate can be injected using the injector6. Much of the injected fuel combusts over the oxidation catalyst 15 orin the DPF 10, removing a portion of the oxygen from the exhaust andreleasing heat.

Even though the same amount of heat is generated, overheating of thereformer 12 can be reduced. If the DPF 10 is not fully heated, as induring denitrations when soot combustion is not also being carried out,the heat can be stored in the DPF 10 and slowly released. If the DPF 10heats to a steady state temperature, as during a prolonged desulfation,a greater portion of the total heat generated is lost to thesurroundings upstream of the reformer 12. That heat that does reach thereformer 12 from the DPF 10 is less problematic than if it weregenerated in the reformer 12 in that the heat is more evenlydistributed. Overheating tends to occur in local hot spots.

The concept of controlling the reformer temperature by using selectivedistribution of fuel between two injection points can be implementedwithout the DPF 12 using, for example, a system as illustrated in FIG. 4where there is an oxidation catalyst 15 between the upstream point offuel injection and the reformer 12. The fuel required by the reformer 12in FIG. 4 can be selectively distributed between the fuel injectors 6and 7. In order to facilitate temperature control by this method, theoxidation catalyst 15 can be specifically designed to readily lose heatto the surroundings. Such a design may involve a high external surfacearea and conductive rather than insulating packaging.

This same concept can be applied, perhaps with even greater effect, tocontrolling the temperature of the DPF 10. FIG. 5 provided an exemplarypower generation system 50 for implementing this concept. To begin DPFregeneration in the system 50, the DPF 10 can be heated by injectingreductant through the injector 6. The reductant combusts in theoxidation catalyst 15, generating heat that warms the DPF 10.

Once soot combustion reaches a self sustaining temperature, the fuelinjection optionally ceases, however, if soot combustion threatens tooverheat the DPF 10, fuel injection can counter-intuitively beincreased. Rather than aggravating the overheating problem, thismitigates the problem. The injected fuel combusts in the oxidationcatalyst 15, removing oxygen from the exhaust. This oxygen is no longeravailable in the DPF 10, thus reducing the soot combustion rate. Thesame total amount of heat may be released, but the distribution issignificantly different. The heat may temporarily reside in theoxidation catalyst 15 and the DPF 10 may have already begun to cool bythe time the heat is transferred downstream. There will be greater heatlost to the surroundings upstream of the DPF 10 due to the highertemperatures. Finally, soot combustion tends to occur along narrowfronts. Whereas the heat produced from these fronts is concentrated, theheat transferred to the DPF 10 through the exhaust is rather evenlydistributed. If temperatures in the DPF 10 can be effectivelycontrolled, a less expensive substrate can be used resulting insignificant cost savings. In particular, cordierite can be used insteadof the more expensive SiC.

FIG. 6 provides another power generation system 60 in which the conceptof limiting DPF temperatures using reductant injection can beimplemented. The power generation system 60 contains several bricksbetween the upstream injector 6 and the DPF 10. These bricks include thereformer 12, the thermal mass 13, the LNT 11, and the SCR catalyst 14.Soot combustion can be initiated using the downstream injector 7. Whensoot combustion rates become too high, reductant injection through theinjector 6 can commence. The fuel injected upstream will be combusted inthe reformer 12, consuming oxygen from the exhaust, but causing littleheating of the DPF 10 due to the thermal mass of the reformer 12 and thevarious intervening devices.

The concept of limiting the DPF temperature using upstream reductantinjection can be implemented with either feedback or feed forwardcontrol. Feed back control involves the use of one or more temperaturesensors, like the sensor 3. If the sensor readings are subject tosignificant delays, it may be desirable to correct them by extrapolationor some other method to obtain and estimate of the current temperature.A typical feedback control strategy is PID control, with the degree offueling tending to increase in proportion to the extent to which a DPFtemperature is in excess of a target maximum.

In some cases, feed forward control may be more desirable. One reason touse feed forward control is that hot spots may tend to occur locally inthe DPF 10, with the hot spots moving as soot combustion progresses.Because the hot spots are local and not always in the same location, itmay be inadequate to rely on sensors. On the other hand, a model,particularly one that is corrected using some sensor data, can predictlocal hot spots.

Another option is to simply control the fuel injection according to apre-determined program. A characteristic of such a program implementingthe inventors' concepts is that the injection rate through the injector6 is maintained or increased after soot combustion within the DPF 10 hasreached a self-sustaining rate. In an exemplary program using just onefuel injector, the reductant injection begins at a rate designed to heatthe DPF 10. After soot combustion begins, the rate is maintained. Assoot combustion completes, the rate of reductant injection is graduallydecreased.

In an exemplary method using two fuel injectors, fuel injection beginsimmediately upstream of the DPF 10 at a rate designed to heat the DPF10. After soot combustion has reached a self-sustaining rate, fuelinjection immediately upstream of the DPF 10 ceases. After sootcombustion has begun in the DPF 10, but before soot combustion reachesan excessive rate, fuel injection begins at a point further upstream,whereby combustion significantly reduces the flow rate of oxygen intothe DPF 10. The upstream fuel injection may be maintained until the DPF10 is nearly regenerated.

Additional measures may be used to limit the exhaust oxygen flow rate.Examples of such measures include increasing exhaust gas recirculation(EGR), throttling the engine air intake, and shifting gears to make theengine run at lower speeds. All of these methods can be used togetherwith the inventors' concepts to limit the rate of soot combustion andheating in the DPF 10.

Another related concept involves placing an oxidation catalyst upstreamof a fuel reformer or an oxidation catalyst-containing DPF. An examplewith a fuel reformer is the power generation system 70 schematicallyshown in FIG. 7. The power generation 70 comprises a diesel engine 9, anoxidation catalyst 15, a reductant injector 7, a fuel reformer 12, a LNT11, and a SCR catalyst 14.

Although the reformer 12 itself contains an oxidation catalyst, theupstream oxidation catalyst 15 can perform several functions. Onefunction is to reduce the oxygen content of the exhaust by combustinghydrocarbons also contained in the exhaust. As in the other concepts,the same amount of heat is generated, but at a point displaced from thereformer 12. During regeneration of the LNT 11, the engine 9 can beoperated to provide additional hydrocarbon to augment this function. Inaddition to simply injecting the hydrocarbons, the engine 9 can provideadditional hydrocarbon by operating near or beyond the smoke limit.

Additional hydrocarbons may also be provided as a natural consequence ofother measures used to reduce the oxygen concentration of the exhaustduring regeneration of the LNT 11. Examples of such measures may includeincreasing EGR rates, throttling the engine air intake, and shiftinggears to reduce the engine speed. Additional hydrocarbons can also beprovided by increasing engine fueling rates. These additionalhydrocarbons can be combusted in the oxidation catalyst 15 while causingonly attenuated heating of the reformer 12

The oxidation catalyst 15 can perform additional functions as well. Onesuch function is that it can operate to heat the exhaust slightly evenwhen the LNT 11 is not being regenerated. This additional heat canextend the operating temperature range of the reformer 12, allowing thereformer 12 to be started at lower exhaust temperatures. This functioncan be facilitated by placing the oxidation catalyst 15 as close to theengine as possible, whereby the exhaust will keep the catalyst 15 atrelatively higher temperatures.

A further advantage of catalyzing or fueling combustion to remove someof the exhaust oxygen upstream of the reformer 12 is that it facilitatesmore precise control of the reformer 12. The oxygen concentration orlambda value of the exhaust between the oxidation catalyst and thereformer can be measured for use in this control. Because less oxygenneeds to be removed, the fuel dose immediately upstream of the reformer12 is smaller and can be controlled more accurately.

Another potential use for the oxidation catalyst 15 is converting NO toNO₂. The NO₂ can function to remove carbonaceous deposits from thereformer 12 and the LNT 11. Increasing the proportion of the NO₂ in theexhaust can also enhance NOx removal by the LNT 11 and the SCR catalyst14. Generally, more catalyst is required to effectuate the NO to NO₂function than the more basic hydrocarbon oxidation function.

The upstream oxidation catalyst is also useful when the reformer 12 isin a separate branch from the main exhaust line. FIG. 8 provides aschematic illustration of a power generation system 80 having this typeof branching. In the system 80, the fuel reformer 12 is in the mainexhaust line, but it is essentially the same if the fuel reformer 12 isin the bypass line 81 and the main exhaust line in parallel with thebranch is empty.

The power generation system 80 is designed without exhaust systemvalves. An exhaust system valve or damper can be used to control thedistribution of exhaust between branches. Such control is desirable interms of limiting fuel penalty, but exhaust treatment systems withexhaust valves may be less reliable than exhaust treatment systemswithout valves.

An additional improvement that is applicable to several of theabove-described concepts is to place a fuel injector in an exhaustmanifold upstream of a turbocharger. FIG. 9 provides a schematicillustration of an exemplary power generation system 90 implementingthis concept. The engine 9 operates to produce exhaust which passesthrough the exhaust manifold 5 to the exhaust line 16. The exhaust line16 contains a fuel reformer 12 and a LNT 11, although the concept is notlimited to these exhaust line components. The manifold containsturbocharger 91, which is configured to provide pressurized air to theinlet 93 of the engine 9. An injector 92 is configured to inject areductant into the manifold 5 upstream of the turbocharger 91.

One advantage of manifold reductant injection is that the reductantundergoes intense mixing with the exhaust as it pass through theturbocharger 91. Thorough mixing promotes better utilization of thereductant in downstream devices, which is particularly important if thereductant is diesel fuel. The types of devices that can benefit fromthis mixing include fuel reformers, oxidation catalysts, and DPFs.

Another advantage is that the exhaust is hotter upstream of theturbocharger 91. At these higher temperatures, the reductant can undergoreactions. In the case of diesel fuel, these reactions include crackingof the diesel fuel into smaller and more reactive molecules. Thesereactions generally involve expansion of the gases and the release ofheat. On the one hand, these reactions can provide a boost to theturbocharger 91. On the other hand, these reactions can release heat andconsume oxygen, thus displacing heat from a downstream device as is donewith the oxidation catalyst 15 in FIG. 5. The function is also similarto the oxidation catalyst 15 in FIG. 5 in that the release of heat andthe production of smaller more reactive reductant molecules canfacilitates low temperature start-up of downstream devices, such as aDPF 10 or a reformer 12.

If the first downstream device is a DPF 10, the use of the manifold fuelinjector 92 can reduce the amount of oxidation catalyst required. TheDPF 10 may be loaded with oxidation catalyst to allow light-off throughthe addition of fuel that combusts in the DPF 10. The same oxidationcatalyst can promote soot combustion. By reducing the amount ofcatalyst, not only can the cost be reduced, but also soot combustionrates and problems with excessively high DPF temperatures during sootcombustion.

A potential problem with the manifold injector 90 and other exhaustsystem injectors is coking. Coke can form from residual fuel left in theinjector when the injector is off, particularly if the injector is offfor long periods of time during which it is subject to hightemperatures. Coke can deteriorate injector performance and causefailures.

FIG. 10 illustrates a fuel injector 100 adapted to implement an airpurge method to the coking problem. The fuel injector 100 is showninstalled within a wall 101 of an exhaust passage, which may be anexhaust line or an exhaust manifold. The fuel injector 100 comprises avalve body 102, a needle 103, a solenoid 104 for controlling theposition of need 103, a fuel source 105, a valve 106 for controlling thefuel flow from the fuel source 105, an air supply 107, and a valve 108for controlling the flow of air from the air supply 107.

When fuel injection is required, the valve 106 opens to admit fuel fromthe fuel source 105. Optionally, the valve 106 is opened and closedrapidly in a controlled manner to regulate the fuel flow rate. Once thefuel dosing is complete and the valve 106 is finally closed, valve 108is opened briefly to admit air from air supply 107. The air flowsthrough the valve body 102, flushing the passages therein of fuel,whereby little or no fuel remains to form coke.

The air supply 107 can be any suitable source of pressurized air.Examples of pressurized air sources are an air pump, an intake manifoldpressurized by a turbocharger, and an exhaust manifold upstream of aturbocharger (providing the injector 100 is not itself installed in anexhaust manifold). In a preferred embodiment, the pressurized air isdrawn from a truck braking system.

The fuel supply 105 can be any suitable source of fuel. A standard fuelpump can be used to obtain fuel from a vehicle fuel tank. To promoteatomization, vaporization, and mixing, however, it can be desirable toobtain higher pressures than the 3 to 6 bars provided by a standardelectric fuel pump. To obtain higher pressures, it is preferred to use apressure intensifier.

FIGS. 11A and 11B illustrates a pressure intensifier 110. The pressureintensifier contains a body 111 and a piston 112 defining an upperchamber 113, a middle chamber 114, and a lower chamber 115. Theintensifier is charged with low pressure fuel from pump 116 asillustrated in FIG. 11A by opening valve 117 and 118 and closing valve119 an 121. Fuel enters the lower chamber through valve 118, forcing thepiston 112 to rise, forcing fuel out of the upper chamber 113, throughvalve 117, to a reservoir from which the pump 116 draws fuel.

Fuel is expelled at high pressure through valve 120 as illustrated inFIG. 11B by closing valves 117 and 118 and opening valves 119 and 121.The pump 116 pumps fuel into the upper chamber 113 through the valve119. The fuel in the upper chamber acts on the piston 112 to force fuelout of the lower chamber 115 through the valve 121. Because the area ofthe upper surface of the piston 112, which is acted on by the fuel inthe upper chamber 113 at the pump pressure, is greater than the area ofthe lower surface of the piston 112, which acts on the fuel in the lowerchamber 115, the fuel in the lower chamber 115 can be pressurized inproportion to the difference in area. Preferably, the fuel ispressurized by at least a factor of 2, more preferably by at least afactor of three. The middle chamber 114 accumulates fuel slippingbetween the piston 112 and the walls of the body 111. The accumulatedfuel is returned to the pump reservoir through port 122

In addition to coking, manifold and exhaust system fuel injectors may besusceptible to overheating. One method to avoid overheating is toprovide an excess fuel flow to the fuel injector. The excess fuel flowis returned to the fuel reservoir, carrying away heat. Circulating fuelin this manner also prevents coking. The fuel flow can be maintained aslong as the injector is subject too high temperatures.

FIG. 12 illustrates and exemplary fuel injector 120 designed toaccommodate an excess fuel flow. The fuel injector 120 comprises a valvebody 125 having internal passages 126, a needle 123, a solenoid 124 forcontrolling the position of need 123, the fuel source 105, an exhaustport 127, and a check valve 128 for controlling the flow of fuel throughthe exhaust port 127. The fuel injector 120 is shown installed within awall 101 of an exhaust passage.

The check valve 128 can be set just below the pressure of the fuelsource 105, whereby there is a continuous fuel flow through the valve127 and the internal passages 126 when the needle valve 123 is closed.This fuel is returned to a reservoir for the fuel source 105. When theneedle valve 123 is open, the pressure drops and the flow is primarilyout the opening created by needle valve 123.

While the engine 9 is preferably a compression ignition diesel engine,the various concepts of the inventor are applicable to power generationsystems with lean-burn gasoline engines or any other type of engine thatproduces an oxygen rich, NOx-containing exhaust. For purposes of thepresent disclosure, NOx consists of NO and NO₂.

The power generation system can have any suitable types of transmission.A transmission can be a conventional transmission such as acounter-shaft type mechanical transmission, but is preferably a CVT. ACVT can provide a much larger selection of operating points than aconventional transmission and generally also provides a broader range oftorque multipliers. The range of available operating points can be usedto control the exhaust conditions, such as the oxygen flow rate and theexhaust hydrocarbon content. A given power demand can be met by a rangeof torque multiplier-engine speed combinations. A point in this rangethat gives acceptable engine performance while best meeting a controlobjective, such as minimum oxygen flow rate, can be selected.

In general, a CVT will also avoid or minimize interruptions in powertransmission during shifting. Examples of CVT systems includehydrostatic transmissions; rolling contact traction drives; overrunningclutch designs; electrics; multispeed gear boxes with slipping clutches;and V-belt traction drives. A CVT may involve power splitting and mayalso include a multi-step transmission.

A preferred CVT provides a wide range of torque multiplication ratios,reduces the need for shifting in comparison to a conventionaltransmission, and subjects the CVT to only a fraction of the peak torquelevels produced by the engine. This can be achieved using a step-downgear set to reduce the torque passing through the CVT. Torque from theCVT passes through a step-up gear set that restores the torque. The CVTis further protected by splitting the torque from the engine, andrecombining the torque in a planetary gear set. The planetary gear setmixes or combines a direct torque element transmitted from the enginethrough a stepped automatic transmission with a torque element from aCVT, such as a band-type CVT. The combination provides an overall CVT inwhich only a portion of the torque passes through the band-type CVT.

A fuel reformer is a device that converts heavier fuels into lightercompounds without fully combusting the fuel. A fuel reformer can be acatalytic reformer or a plasma reformer. Preferably, the reformer 12 isa partial oxidation catalytic reformer comprising a steam reformingcatalyst. Examples of reformer catalysts include precious metals, suchas Pt, Pd, or Ru, and oxides of Al, Mg, and Ni, the later group beingtypically combined with one or more of CaO, K₂O, and a rare earth metalsuch as Ce to increase activity. A reformer is preferably small in sizeas compared to an oxidation catalyst or a three-way catalyst designed toperform its primary functions at temperatures below 450° C. The reformeris generally operative at temperatures from about 450 to about 1100° C.

The LNT 11 can comprise any suitable NOx-adsorbing material. Examples ofNOx adsorbing materials include oxides, carbonates, and hydroxides ofalkaline earth metals such as Mg, Ca, Sr, and Ba or alkali metals suchas K or Cs. Further examples of NOx-adsorbing materials includemolecular sieves, such as zeolites, alumina, silica, and activatedcarbon. Still further examples include metal phosphates, such asphosphates of titanium and zirconium. Generally, the NOx-adsorbingmaterial is an alkaline earth oxide. The absorbent is typically combinedwith a binder and either formed into a self-supporting structure orapplied as a coating over an inert substrate.

The LNT 11 also comprises a catalyst for the reduction of NOx in areducing environment. The catalyst can be, for example, one or moretransition metals, such as Au, Ag, and Cu, group VIII metals, such asPt, Rh, Pd, Ru, Ni, and Co, Cr, or Mo. A typical catalyst includes Ptand Rh. Precious metal catalysts also facilitate the adsorbent functionof alkaline earth oxide absorbers.

Adsorbents and catalysts according to the present invention aregenerally adapted for use in vehicle exhaust systems. Vehicle exhaustsystems create restriction on weight, dimensions, and durability. Forexample, a NOx adsorbent bed for a vehicle exhaust systems must bereasonably resistant to degradation under the vibrations encounteredduring vehicle operation.

The ammonia-SCR catalyst 14 is a catalyst effective to catalyzereactions between NOx and NH₃ to reduce NOx to N₂ in lean exhaust.Examples of SCR catalysts include oxides of metals such as Cu, Zn, V,Cr, Al, Ti, Mn, Co, Fe, Ni, Pd, Pt, Rh, Rd, Mo, W, and Ce, zeolites,such as ZSM-5 or ZSM-11, substituted with metal ions such as cations ofCu, Co, Ag, Zn, or Pt, and activated carbon. Preferably, the ammonia-SCRcatalyst 14 is designed to tolerate temperatures required to desulfatethe LNT 11.

Although not illustrated in any of the figures, a clean-up catalyst canbe placed downstream of the other aftertreatment device. A clean-upcatalyst is preferably functional to oxidize unburned hydrocarbons fromthe engine 9, unused reductants, and any H₂S released from the NOxabsorber-catalyst 11 and not oxidized by the ammonia-SCR catalyst 14.Any suitable oxidation catalyst can be used. To allow the clean-upcatalyst to function under rich conditions, the catalyst may include anoxygen-storing component, such as ceria. Removal of H₂S, where required,may be facilitated by one or more additional components such as NiO,Fe₂O₃, MnO₂, CoO, and CrO₂.

The invention as delineated by the following claims has been shownand/or described in terms of certain concepts, components, and features.While a particular component or feature may have been disclosed hereinwith respect to only one of several concepts or examples or in bothbroad and narrow terms, the components or features in their broad ornarrow conceptions may be combined with one or more other components orfeatures in their broad or narrow conceptions wherein such a combinationwould be recognized as logical by one of ordinary skill in the art.Also, this one specification may describe more than one invention andthe following claims do not necessarily encompass every concept, aspect,embodiment, or example described herein.

1. A method of operating a diesel power generation system, comprising:operating a compression ignition diesel engine to produce a lean exhaustcomprising NO_(x) and particulate matter; treating the exhaust bypassing it through a fuel reformer, a first DPF, a thermal mass, and alean NO_(x) trap (LNT), whereby the LNT traps a portion of the NO_(x)and the first DPF traps a portion of the particulate matter; determiningwhen to denitrate the LNT; in response to the determination, injectingfuel into the exhaust at a rate that leaves the exhaust lean, wherebythe injected fuel combusts, the fuel reformer heats to steam reformingtemperatures, and the first DPF also heats; after heating the fuelreformer to steam reforming temperatures, denitrating the LNT by makingthe exhaust entering the fuel reformer, the first DPF, and the leanNO_(x) trap rich, whereby the fuel reformer produces reformate thatdenitrates the LNT; and in conjunction with the foregoing steps ofheating the fuel reformer and denitrating the LNT, heating the first DPFto soot combustion temperatures, whereby the first DPF is regeneratedeach time the LNT is denitrated; wherein the first DPF is functional tofilter a substantial portion of the particulate matter from the exhaustand has a low thermal mass that facilitates the first DPF being heatedto soot combustion temperatures each time the LNT is denitrated; whereinthe fuel reformer comprises a steam reforming catalyst and has a lowthermal mass that facilitates its being heated to steam reformingtemperatures for each LNT denitration; wherein the LNT is functional toabsorb and store NO_(x) under lean conditions and reduce the storedNO_(x) and regenerate under rich conditions; and the thermal mass isfunctional to substantially reduce the temperatures to which the LNT isheated during the fuel reformer heating and LNT denitration.
 2. Themethod of claim 1, wherein treating the exhaust further comprisespassing the exhaust through a second DPF downstream from the first DPF.3. The method of claim 2, wherein the second DPF has a high thermal massand functions as the thermal mass.
 4. The method of claim 2, wherein thefirst DPF operates primarily by depth filtration whereas the second DPFoperates primarily by cake filtration.
 5. The method of claim 2, whereinthe second DPF is located downstream from the LNT.
 6. The method ofclaim 2, further comprising: desulfating the LNT; wherein the second DPFis regenerated each time the LNT is desulfated and the LNT is desulfatedeach time the second DPF is regenerated.
 7. The method of claim 2,wherein the second DPF and LNT are sized so that the second DPF needs tobe regenerated to remove accumulated soot approximately as often as theLNT needs to be regenerated to remove accumulated SO_(x).
 8. The methodof claim 1, wherein the first DPF is located upstream from the fuelreformer.
 9. The method of claim 1, wherein: treating the exhaustfurther comprises passing the exhaust through an SCR catalyst locateddownstream from the LNT; wherein the SCR catalyst is functional toabsorb and store ammonia when the LNT is denitrating and to remove asecond portion of the NO_(x) from the exhaust under lean conditions bycatalyzing a reaction between the NO_(x) and the ammonia.
 10. A powergeneration system configured, adapted, and functional to operateaccording to the method of claim
 1. 11. A vehicle comprising the powergeneration system of claim
 10. 12. A diesel power generation system,comprising: a compression ignition diesel engine operative to produce alean exhaust comprising NO_(x) and particulate matter; a first DPF thathas a low thermal mass and is functional to filter a substantial portionof the particulate matter from the exhaust; a fuel reformer comprisingoxidation and steam reforming catalysts and having a low thermal mass;an LNT that is functional to absorb a portion of the NO_(x) from theexhaust and store the NO_(x) under lean conditions and to reduce storedNO_(x) and regenerate under rich conditions; a thermal mass, which has ahigh thermal mass; an exhaust line configured to channel the exhaustfrom the engine through the fuel reformer and the first DPF, then thethermal mass, and then the lean NO_(x) trap; a fuel injector configuredto inject fuel into the exhaust line upstream from the fuel reformer andthe first DPF; a controller functional to determine when to denitratethe LNT and configured to carry out denitration through control over atleast the operation of the fuel injector; wherein the controller isprogrammed to denitrate the LNT by making the determation to denitratethe LNT and in response to that determination, injecting fuel into theexhaust at a rate that leaves the exhaust lean until the fuel reformerhas heated to steam reforming temperatures, and then making exhaustrich; wherein the fuel reformer is designed and configured to be rapidlyheated to steam reforming temperatures; wherein the first DPF isdesigned and configured to regenerate with each LNT denitration byvirtue of being designed and configured to rapidly heat to sootcombustion temperatures as the fuel reformer is heated and the LNT isdenitrated; wherein the thermal mass is configured to and functional tosubstantially reduce the temperatures to which the LNT is heated whenthe LNT is denitrated.
 13. The system of claim 12, further comprising asecond DPF located within the exhaust line downstream from the firstDPF.
 14. The system of claim 13, wherein the second DPF is a wall flowfilter and the first DPF is a flow through filter.
 15. The system ofclaim 13, wherein the first DPF is located upstream from the thermalmass, but downstream from the fuel reformer, and the second DPF islocated downstream from the LNT.
 16. The system of claim 13, wherein thesecond DPF and the LNT are sized so that the second DPF needs to beregenerated to remove accumulated soot approximately as often as the LNTneeds to be regenerated to remove accumulated SO_(x).
 17. The system ofclaim 12, wherein the thermal mass is a second DPF.
 18. The system ofclaim 12, wherein the first DPF is located upstream from the fuelreformer.
 19. The system of claim 12, further comprising: an SCRcatalyst located downstream from the LNT; wherein the SCR catalyst isfunctional to absorb and store ammonia when the LNT is denitrating andto remove a second portion of the NO_(x) from the exhaust by catalyzinga reaction between the NO_(x) and the ammonia under lean conditions. 20.A vehicle comprising the power generation system of claim 12.